In a nonvolatile semiconductor memory device of a floating gate type, as an interpoly dielectric (IPD) for electrically insulating a floating gate (FG) electrode and a control gate (CG) electrode from each other, use is made of an ONO film having a laminated structure composed of three films of a silicon oxide film, a silicon nitride film and a silicon oxide film.
The silicon nitride film in this ONO film does not have a completely stoichiometric composition because of a forming method thereof, and has a composition in which silicon atoms are slightly excessive (in proportion by mole). In such a silicon nitride film, wherein silicon atoms are excessive, a level at which electric charges are trapped exists. Thus, when an electric field is applied to the nonvolatile semiconductor memory device, that is, a data is written or erased in the nonvolatile semiconductor memory device, the level, in the silicon nitride film, traps a part of leak charges from the FG electrode. Moreover, the barrier height of the silicon nitride film is lower than that of the silicon oxide films; therefore, the charges trapped in the silicon nitride film do not easily undergo a detrap (leak) to the outside of the ONO film because of both of the trap level of the silicon nitride film and the barrier height of the silicon oxide films formed to sandwich the silicon nitride film. Thus, the charges trapped in the silicon nitride film make the self-electric-field of the silicon nitride film high. This matter causes a rise in the barrier height of the interface between the FG electrode, and the IPD film, which is the ONO film. As a result, the leak current of the IPD film can be decreased.
Nonvolatile semiconductor memory devices are each desired to be further shrunken. As the shrinkage is advancing, the number of charges stored in the FG electrode in the nonvolatile semiconductor memory device is becoming smaller. Thus, in a case where charges trapped in the silicon nitride film of its ONO film when a data is written or erased are detrapped toward the FG electrode when the charges are retained, an effect given to the charge-retention characteristic of the nonvolatile semiconductor memory device by the detrap is never small. Accordingly, in the light of a tendency that the shrinkage of nonvolatile semiconductor memory devices causes a deterioration in their charge-retention characteristic, it is further required to avoid the matter that the charges trapped in the silicon nitride film of the ONO film are detrapped when the charges are retained.
Furthermore, for nonvolatile semiconductor memory devices, required are not only the shrinkage thereof but also a matter that their ONO film, as their IPD film, is made thinner in order to avoid an increase in interference between their semiconductor memory elements by the shrinkage. However, as the ONO film is made thinner, the leak current is further increased. It is therefore difficult to make the ONO film thinner while the leak current is restrained.